A not so happy ending

Everyone expects a happy ending at the end of a book. An ending where the main character defeated the villain and lives happily ever after. Why do we expect that happy ending? Why do we expect that the main character defeated the villain and lives happily ever after? Why? Why do we like books where the main character goes through some ups-and-downs, but ends up living an almost 'perfect' life? I really don't understand that. So I decided to write a book that's more life-related. Because after all, not everyone has a happy ending where they defeated the villain.